The present invention relates to liquid crystal display (LCD) devices and, more particularly, to an improved method for sealing the peripheral edges of an LCD device to retain the liquid crystal material therein.
One example of an electronic device which is filled with a fluid is an LCD device An LCD device typically comprises a multiplicity of picture elements, or "pixels", formed between a pair of flat panels (usually a glass substrate and a coverglass) sealably containing a quantity of liquid crystal material, such as a dichroic dye guest/host system or a twisted nematic formulation. If the LCD device is to operate by reflected light, only one of the panels need be transparent and the other panel will be formed with a reflective surface. If the LCD device is to be light transmissive, then both flat panels should be transparent.
A detailed description of an LCD device structure and operation is disclosed and claimed in U.S. Pat. No. 4,565,424, issued Mar. 3, 1987, assigned to the assignee of the present invention, and incorporated herein in its entirety by reference. Briefly described, an LCD device may be fabricated by disposing a plurality of electrodes, formed from a transparent conductive material, such as indium tin oxide (ITO) or the like, on one of the flat panels (glass substrate); the electrodes will usually be arranged in uniform columns and rows to form an X-Y matrix structure. These electrodes are generally referred to as "pixel" electrodes. A semiconductor switch, such as a thin-film field-effect transistor (FET) or the like, is integrally formed with each pixel electrode to control operation of that pixel in the display. Electrical communication with the individual pixel FETs is accomplished by a plurality of X-address lines or scan lines and a plurality of Y-address lines or data lines which are both formed during device fabrication. A layer of light transmissive passivation material, such as silicon oxide or the like, is typically formed over each FET and corresponding pixel electrode. The second flat panel (coverglass) will have conductive material disposed on, and substantially completely covering, an inner surface to form a "ground plane electrode." The ground plane electrode will be made from ITO or the like if the display is to be light transmissive. The coverglass will be disposed over the FETs and pixel electrodes with the ground plane electrode facing the FETs and pixel electrodes; the coverglass will typically be spaced from each FET structure by a mechanical spacer such as glass fibers, glass beads or the like disposed over the FET. The mechanical spacer causes a gap between the glass substrate and the coverglass. The glass substrate and coverglass are joined together at the peripheral edges of the device by applying a sealant/adhesive material, such as epoxy or the like, to the peripheral edges of the coverglass and then disposing the coverglass over the substrate in alignment therewith. The sealant material should fill the gap between the glass substrate and the coverglass at the periphery of the device. The device is then heat treated to cure the epoxy. The seal may extend essentially completely around the periphery of the device and an aperture or window may be formed at a corner or in a side edge of the device for later injection of the liquid crystal material. If the seal extends completely around the device periphery to hermetically seal the interior of the device, an opening or window may then be etched through the coverglass and ground plane electrode by known techniques to facilitate injection of the liquid crystal material into the interior of the device. After the epoxy seal is heat cured, the interior of the LCD device is evacuated and then the liquid crystal material is injected with a syringe or the like through the etched opening in the coverglass or through the window in the epoxy seal at a corner or in a side edge of the device. The liquid crystal material will migrate throughout the device and fill the voids between the plurality of pixel electrodes and the ground plane electrode. When the device is filled, the etched opening or side window is closed with an epoxy-type material or the like. These materials have a tendency to partially permeate the liquid crystal material immediately adjacent to the opening or window and thus contaminate this nearby liquid crystal material. This is a particularly troublesome problem when the opening is plugged with an epoxy-type material which is heat cured; the epoxy-type materials typically contain a solvent which has a very high tendency to disperse into the liquid crystal material and contaminate a substantial area of the display immediately around the opening. These contaminants can adversely affect the optical and electrical characteristics of the liquid crystal material in the contaminated area.
This problem becomes exacerbated as the size of liquid crystal display devices increases because the opening through which the liquid crystal material is inserted is typically made larger to facilitate efficient injection of the material during the fabrication process. The method where an opening is etched in the coverglass and ground plane electrode is typically only used for displays smaller than about 4".times.4", because a small opening can be used that does not take up valuable display area; and the other method, where a window or aperture is formed in the peripheral edge seal (preferably at a corner of the device), is used for larger devices because a larger aperture can be formed in the edge seal without reducing the display area. Since the aperture is larger, more epoxy-type material must be used to close it and a larger area of the display proximate to the aperture may become contaminated by solvents and the like. Additionally, the liquid crystal material is preferably filled from a single aperture. If the liquid crystal material were filled through multiple small apertures, which individually would not require much epoxy-type filling material, the liquid crystal cell may be subjected to greater overall contamination.
It is accordingly a primary object of the present invention to provide a novel method for sealing an LCD device or the like which is not subject to the foregoing disadvantages.
It is a further object of the present invention to provide a method for selectively forming, in a localized area, a seal or plug which does not contaminate or otherwise impair the performance of an LCD device.
It is an additional object of the present invention to provide a noncontaminating edge seal for an LCD device which can withstand wide temperature and humidity variations without loss of adhesion or cracking.
These and other objects of the invention, together with the features and advantages thereof, will become apparent from the following detailed specification when read with the accompanying drawings.